The present application is a continuation of PCT/EP01/12883, filed on Nov. 7, 2001, now WO 02/38636 (published in French). The present invention relates to a catalytic system that can be used to prepare by polymerization, diene elastomers comprising polyisoprenes and polybutadienes to a process for the preparation of said catalytic system, and to a process for preparing diene elastomers comprising polyisoprenes having a high cis-1,4 linkage content and polybutadienes using the catalytic system.
It is known that polyisoprenes having a high cis-1,4 linkage content may be prepared using catalytic systems based on:                a rare earth salt in solution in a hydrocarbon solvent,        an alkylating agent of this salt consisting of an alkylaluminium, and        an alkylaluminium halide.        
For example, the document “Report of the Academy of Sciences of the USSR, volume 234, no. 5, 1977 (Y. B. Monakov, Y. R. Bieshev, A. A. Berg, S. R. Rafikov), shows that isoprene may be polymerized using a catalytic system comprising:                a bis(2-ethylhexyl)phosphoric acid salt of neodymium or praseodymium, as the rare earth salt, in solution in toluene,        triisobutylaluminium as the alkylating agent, in an “alkylating agent:rare earth salt” molar ratio of 20, and        diethylaluminium chloride as the alkylaluminium halide.        
Mention may also be made of the document, “Proceedings of China-U.S. Bilateral Symposium on Polymer Chemistry and Physics, Science Press, pp. 382-398, 1981 (O. Jun, W. Fosong, S. Zhiquan)”. This document teaches the use of a bis(2-ethylhexyl)phosphoric acid salt of neodymium, in association with triethylaluminium or triisobutylaluminium, and an alkylaluminium halide of the formula Al2(C2H5)3Cl3.
U.S. Pat. No. 3,794,604 exemplifies a catalytic system of the type which has been “preformed” in the presence of a conjugated diene monomer and comprising:                butadiene or isoprene as the conjugated diene monomer,        cerium octanoate as the rare earth salt in solution in benzene,        diisobutylaluminium hydride as the alkylating agent in an “alkylating agent:rare earth salt” molar ratio substantially equal to 20 and        ethylaluminium dichloride as the alkylaluminium halide.        
It will be noted that the only polymerization examples stated in this patent relate to the polymerization of butadiene.
Japanese patent specification JP-A-60/23406 also describes a catalytic system of the type which has been “preformed” in the presence of butadiene, the system specifically being intended for the polymerization of butadiene. The catalytic systems tested in the examples of embodiment of said document comprise:                a bis (2-ethylhexyl) phosphoric acid salt of neodymium as the rare earth salt in solution in n-hexane or cyclohexane,        triisobutylaluminium or diisobutylaluminium hydride as the alkylating agent in an “alkylating agent:rare earth salt” molar ratio ranging from 10 to 30, and        ethylaluminium sesquichloride as the alkylaluminium halide.        
It will be noted that none of the polybutadienes, obtained by means of these catalytic systems, simultaneously exhibit a Mooney viscosity ML(1+4) at 100° C. greater than or equal to 40 and a polydispersity index of below 2.5. As a result, these polybutadienes are not suitable for use in a tire tread.
Another major drawback of these known catalytic systems is that they exhibit differing levels of activity for the polymerization of the various conjugated dienes, in particular for the homopolymerization of either isoprene or butadiene.
Another drawback is the non-reproducible nature of the macrostructural and microstructural properties exhibited by the polymers obtained by means of these catalytic systems, in particular with regard to the content of cis-1,4 linkages, which may vary significantly.